Exploitation of natural gas hydrates involves, first, obtaining a core sample of natural gas hydrates by drilling and sampling, analyzing the core sample, and assessing hydro-geological parameters such as storage of the natural gas hydrates, and occurrence, scale and property of ore beds of the natural gas hydrate. Thus, drilling and sampling are the most direct measures for exploitation of natural gas hydrates. Natural gas hydrates exists in sedimentary strata with the temperature thereof is 0 to 10° C. and the pressure thereof is higher than 10 MPa, and the natural gas hydrates dissociates in the condition that the temperature of the strata containing the natural gas hydrates increase or that the pressure of the strata decrease. Substantial heat is generated by a drill bit. In this way cutting rocks during drilling and sampling construction, and heat is generated by friction of a drilling tool and a wall of a drilling hole, both of which cause the temperature at the bottom of the hole to increase. The temperature of drilling mud increases as the heat is transferred to the mud. The increasing of the mud temperature will lead to the natural gas hydrates to dissociate during drilling the core of the natural gas hydrates, which makes it possible that no in-situ hi-fi core sample of the natural gas hydrates be obtained. This may not only affects the assessment of the storage of ore bed, but also cause accidents in the drilling hole and damage drilling equipments used. Consequently, the temperature of low temperature mud used for drilling must be controlled and should be generally maintained in the range of −3° C. to 3° C., in order to ensure the stability of the natural gas hydrate stratum and core during drilling.
At present, techniques for cooling mud have been developed. The ground temperature is up to 350° C. and the temperature of the returned mud is up to 60 to 111° C. during drilling in hot water layer of deep geothermal well. The highest ground temperature in WD-1A well in Kakkonda, Japan is up to 500° C., and the mud with high temperature causes severe corrosions to drilling tools and tubes and scalds operation staff easily. Lengthening circulating path of mud channels is generally adopted for cooling the mud, so that the returned mud can be cooled down naturally during the circulation. Another method adopted is adding ice to a mud pond to lower the mud temperature, and mud cooling devices can be deployed if necessary. The designed mud cooling devices include: a cooling tower mounted to the mud pond, and a power fan mounted near a vibrating screen for forced cooling. However, all the above techniques are for high temperature mud, and they are not suitable for cooling low temperature mud sampled during natural gas hydrates drilling, the mud temperature of which should be controlled within the range of −3° C. to 3° C.